An insulated gate bipolar transistor (IGBT) is a three-terminal power semiconductor device with a low voltage drop, high efficiency and fast switching operational characteristics. The IGBT is used to switch electric power in many modern appliances. The IGBT combines the gate-drive characteristics of a metal oxide semiconductor field effect transistor (MOSFET) with the high current and low saturation voltage capability of bipolar transistors by combining an isolated gate FET for the control input and a bipolar power transistor as a switch in a single device.
FIGS. 1a and 1b illustrate a conventional trench IGBT structure. FIG. 1a is a top partially cut away view of the conventional trench IGBT structure, and FIG. 1b is a cross-sectional view of the conventional trench IGBT structure taken along line A-A'. The trench IGBT structure includes a collector electrode 100. A P+ layer 110 is formed above the collector electrode 100, and an N layer 120 is formed above the P+ layer 110. An N− layer 130 is formed above the N layer 120, and P wells 140 are formed above the N− layer 180. A trench is formed between adjacent P wells 140 and extends into the N− layer 130 such that the adjacent P wells 140 are isolated from each other. An N+ source region 150 is formed in each upper corner of each P well 140 proximate the trench. A gate 160 is formed in each trench. The gate 160 is surrounded by an oxide layer 170 such that the oxide layer 170 is provided between the gate 160 and the N− layer 130, the P wells 140 and the N+ source regions 150. An emitter electrode 180 is formed above the P wells 140, the N+ source regions 150 and the gate 160 including the oxide layer 170.
The conductivity of a semiconductor may be varied in proportion to the density of charge carriers. For example, increasing the amount of charge carriers increases the conductivity of the device. This phenomenon is referred to as “conductivity modulation.” FIG. 1c illustrates a graph of charge carrier densities (i.e., electron and hole densities) in the N− layer 130 and the N layer 120 of the conventional trench IGBT structure shown in FIGS. 1a and 1b. 
In operation, when a positive voltage is applied to the gate 160, electrons move from the N+ source regions 150 and the P wells 140 into the N− layer 130 and the N layer 120. However, the electrons do not easily traverse the junction barrier between the N layer 120 and the P+ layer 110, resulting in increased electron density at the barrier. In the opposite direction, holes from the P+ layer 110 move into the N layer 120 and the N− layer 130. The holes in the N− layer 130 easily traverse the junction and into the P wells 140 which decreases the charge carrier density at the junction. Thus, the conventional IGBT structure causes many holes to escape.
In order to limit the amount of holes that escape the N− layer 130 and enter the P wells 140, a width of the gate 160 may be increased to create a wide trench IGBT. A wide trench IGBT structure enhances conductivity modulation because a bottom of the wide trench blocks hole escape through the P wells 140 and also supplies electrons to combine with the holes. The electrons compensate for the positive charge of the holes. The density of electrons and holes can be increased under the wide trench. As a result, the wide trench IGBT structure promotes conductivity modulation of the N layer 120 and the N− layer 130 of the device. However, conventional wide trench IGBTs may be structurally unstable and mechanically weak, and are also difficult to manufacture.
Therefore, it is desirable to provide an improved wide trench IGBT structure.